The monad-exception package

Extensible exceptions are a good solution to the exception problem in
Haskell. However, there is one problem: they are not extensible enough!
The problem is that the functions defined in Control.Exception for dealing
with exceptions can only be used with the IO monad. A lot of Haskell code
uses a stack of monads, at the bottom of which is IO, but the IO monad
is not used directly.

There have been many attempts to solve this problem, but the stumbling block
has been the presence of short-circuiting monad transformers: sometimes,
these prevented the cleanup actions from being run, making it effectively
impossible to catch exceptions in such monads. The monad-control package
has been developed as a solution to this problem: it defines a way to turn
a monad transformer stack "inside-out", which ensures that cleanup actions
are run even when the original action short-circuits. The lifted-base
package, built on top of monad-control, exports the
Control.Exception.Lifted module, which contains versions of the
Control.Exception functions that work on any monad stack with IO at its
base.

This has pretty much solved the above problems. However, one thing that the
solutions that came before monad-control did was provide a type class
encapsulating exception functionality that could be implemented by pure
monads, allowing you to use the same interface to throw and catch exceptions
in both pure and IO-based code. This also makes it possible to express
which can throw an exception, but which don't necessarily do any IO and
which are polymorphic in their exception throwing (i.e., you could run the
function in IO and it would use throwIO, or you could run it as an
Either and it would use Left).

That's what this package does. It provides a MonadException type class (in
the Control.Monad.Exception.Class module), which has instances for IO
and IO-like monads (for which monad-control is used to provide the
correct instances as described above), as well as for some pure monads.
Several overlapping instances (in the spirit of mtl-evil-instances) are
provided, so it is not necessary to provide a pass-through instance for
MonadException for every monad transformer you write.

This package also defines an ExceptionT monad transformer (in
Control.Monad.Trans.Exception) that can be used to add MonadException
functionality to otherwise pure monad stacks. mtl-evil-instances is used
to automatically provide pass-through instances for the mtl type classes
for this transformer.

Finally, this package includes the module Control.Exception.Monadic, which
is a full replacement for Control.Exception, whose functions work on
any instance of MonadException and not just IO. The functions for
dealing with asynchronous exceptions require IO however, so these are only
polymorphic for any IO-like monadic (as determined by monad-control).